The present invention relates to a one-piece multipole plate, preferably but without limitation for a magnetic clamping apparatus and a process for making such plate.
As used herein, the term magnetic clamping apparatus is intended to indicate:                a permanent-magnet apparatus, i.e. an apparatus that does not require any power supply when used for clamping or for changing its state from active to inactive and vice versa, and is formed with permanent magnets in appropriate arrangement within the apparatus;        an electro-permanent apparatus, i.e. an apparatus that does not require any power supply when used for clamping and requires power supply when it is activated and inactivated, and is formed with reversible permanent magnets and, if needed, with static permanent magnets in appropriate arrangement within the apparatus;        an electromagnetic apparatus, i.e. an apparatus that requires power supply when used for clamping, whose magnetic core is made of ferromagnetic material.        
In prior art, also with reference to FIGS. 1A and 1B, the process for making a magnetic clamping apparatus 1 for example of the electro-permanent dual-magnet type, includes a first step in which a frame is formed from solid ferromagnetic material, in which a number “N” of coils 3, also known as solenoids, are arranged.
Otherwise, the frame 2 may be formed by assembling together various components with methods well known to those skilled in the art.
The solenoids 3 are appropriately arranged to obtain North/South polarities and are electrically, connected with a power source located outside the frame 2 (not shown).
The solenoids 3 have such a configuration as to define a space for receiving a reversible magnet 4, such as a magnet of the AlNiCo type, above which a pole piece 5 is placed.
The pole piece 5 is obtained by mechanically machining solid ferromagnetic material.
It shall be noted that, as used herein, the term pole piece is intended to indicate an element formed of ferromagnetic material that typically has a surface that is magnetically neutral when the magnetic apparatus is not activated and magnetically active when the magnetic apparatus is activated.
In the particular representation of FIG. 1B, the pole piece 5 is shown as a ferromagnetic element having a square plan section with six faces of given width, length and thickness.
Particularly, the pole piece 5 has four of its six faces in which the magnetic field is oriented in one direction, a fifth face in which the direction of the magnetic field, and thus its North/South polarity, can be changed, and a sixth face 5A that can be neutral when the magnetic apparatus is not activated or have the same polarity as the remaining five faces when the magnetic apparatus is activated.
It shall be noted that the magnetic apparatus 1 comprises a plurality of pole pieces 5, which are physically separated from each other and are coupled to the frame 2 to form a workpiece holding surface 2A, on which the workpieces to be mechanically machined are arranged.
In other words, all the faces 5A of the pole pieces 5 form the holding surface 2A of the magnetic plate of the magnetic apparatus, on which the workpieces to be mechanically machined are arranged and firmly clamped, as the magnetic apparatus is activated.
Then, the process includes the step of associating the pole pieces 5 with the frame 2, for example, by means of a screw 6, so that the solenoid 3-reversible magnet 4 assembly can be clamped into a pack.
For this purpose, to allow each pole piece 5 to be coupled to the frame 2, holes 7 are formed both in the frame 2 and in the pole piece 5, such holes being designed to engage the screw 6 for clamping each pole piece 5 against the frame 2.
Furthermore, pole extensions (not shown) may be respectively associated with one or more pole pieces 5, when specifically needed for machining the workpieces.
The pole extension may be associated with the pole piece 5 of the magnetic apparatus 1, for example, by screw connection of the pole extension in an additional hole 8 formed in the pole piece 5, such hole 8 extending along the same longitudinal axis of the hole 7.
Also, the process includes a step during which a static magnet 9, such as Ferrite or NdFeB, also appropriately oriented, is fitted in the gap between the pole pieces 5.
Finally, the process includes a “calibration” step, in which the flux of the reversible magnet 4 is balanced with the flux of the static magnet 9 and a resin casting step 10, whereby the magnetic apparatus 1 can be made substantially impervious to impurities and/or liquid infiltrations, and any gaps can be filled.
Nonetheless, this process for making the apparatus 1 still suffers from certain problems, including the ones associated with the calibration step.
In addition to being time consuming, the calibration step has to be carried out by specially skilled persons.
It shall be noted that the calibration step is required due to certain problems specially associated with the magnetic apparatus 1, such as:
a) the total flux value that can be obtained from the static magnets of each pole piece, even when it is statistically calculated beforehand, may differ from the value of the reversible magnet being used, in terms of quality, quantity, etc.;
b) the center-to-center distances between each pair of pole pieces 5 that form the holding surface of the magnetic, plate, as well as the distances between the faces of each pair of pole pieces 5 can change due to dimensional tolerances of the various materials (static magnet, pole pieces);
c) the faces of each pair of pole pieces 5 between which the static magnet 9 is fitted are non-parallel due to the screw connection of the pole piece with the frame.
In addition to the above, more problems are associated with the fabrication of a magnetic apparatus, whether or not it is of electro-permanent dual-magnet type, such as:                the impossibility of achieving accurate alignment and equal spacing of the holes 8 formed in the top of the pole pieces 5;        the poor ability of pole pieces 5 of absorbing the vibrations caused by mechanical machining of workpieces held against the clamping plate, particularly when pole extensions are used;        the above vibrations can cause the filling resin 10 to break, and allow the cooling liquids to infiltrate to the solenoid area 3 and cause a short-circuit.        